This invention relates to a process for the chlorination of toluene to form a monochlorotoluene product having a substantially reduced proportion of orthochlorotoluene to parachlorotoluene.
It is known that the presence of an alkyl group on an aromatic ring tends to direct incoming substituents in electrophilic aromatic substitution reactions to the ortho- and para- positions of the aromatic ring. Thus, for example, in the chlorination of toluene, the usual product is a mixture of orthochlorotoluene and parachlorotoluene. The presence of two ortho- ring positions and only a single para- ring position results in the formation of a considerable excess of orthochlorotoluene over parachlorotoluene, often in proportions approximating the theoretical 2:1 ratio of orthochloro:parachloro isomer.
The chlorination of toluene is frequently carried out in the presence of a Lewis acid catalyst such as ferric chloride or antimony chloride. The monochlorotoluene produced in such a process will generally contain a significant excess of orthochlorotoluene over parachlorotoluene. From a commercial point of view, the production of orthochlorotoluene in such a process is undesirable since little, if any, demand for that isomer exists. On the other hand, the parachloro isomer is readily marketable and is a commercially valuable product. Thus, the orthochlorotoluene must be stored, disposed of in a non-polluting manner or recycled by hydrogenation to toluene. Such measures markedly reduce the efficiency and significantly increase the expense of the toluene chlorination process. It will be apparent therefore that it is desirable, from a commercial point of view, to chlorinate toluene under conditions which minimize the formation of orthochlorotoluene and maximize the formation of parachlorotoluene. Processes to achieve this purpose are known in the prior art. Thus, for example, it is known from U.S. Pat. No. 1,946,040 that when alkylbenzenes are reacted with chlorine, the yield of parachlorinated product is improved with the aid of a mixed catalyst comprising sulfur and antimony trichloride and optionally, iron or lead. In British Pat. No. 1,153,746 (1969) it is disclosed that in the chlorination of toluene in the presence of a ring chlorination catalyst, such as iron, ferric chloride, antimony chloride and the like, the ratio of orthochloro to parachloro isomers produced may be lowered by the presence of an organic sulfur compound such as thiophene, hexadecylmercaptan, dibenzothiophene or the like. Furthermore, in British Pat. No. 1,163,927 (1969) it is disclosed that the proportion of parachlorotoluene produced may be improved when toluene is chlorinated in the presence of elemental sulfur or an inorganic sulfur compound and a ring-chlorination catalyst such as ferric chloride, aluminum chloride, antimony chloride, and the like. The processes thus disclosed provide a substantial improvement in the yield of parachlorotoluene. Nevertheless, it will be apparent that still further improvement in the yield of parachlorotoluene is desirable. In U.S. Pat. No. 3,226,447, issued Dec. 28, 1965 to Bing et al, it is disclosed that in the substitution - chlorination of benzenes and toluene by chlorine, the ratio or ortho isomer to para isomer in the chlorinated product may be lowered when the reaction is carried out in the presence of an iron, aluminum, or antimony halide catalyst and a co-catalyst which is an organic sulfur compound wherein the sulfur is divalent. Examples of such co-catalyst include various mercaptans, mercapto-aliphatic carboxylic acids, aliphatic thiocarboxylic acids, alkyl sulfides, alkyl disulfides, thiophenols, aryl sulfides, aryl sulfides, aryl disulfides and the like containing divalent sulfur. The use of such co-catalysts in the chlorination of toluene produces a product wherein the ratio of orthochlorotoluene to parachlorotoluene is 1.2, indicating a considerable improvement over the ortho to para isomer ratio achieved in the absense of the co-catalyst. However, it will be apparent that even a 1.2 ratio of ortho to para isomer represents a considerable economics disadvantage in the production of substantial amounts -- greater than 50 percent of the monochlorotoluene mixture -- of the unwanted ortho isomer. Thus, it will be apparent that a considerable commercial benefit is to be derived from a still further lowering of the ortho to para isomer ratio.
Accordingly, it is an object of this invention to provide a process for the chlorination of toluene whereby the proportion of orthochlorotoluene to parachlorotoluene product is substantially reduced. It is a further object to provide a novel catalyst system capable of exerting a paradirecting effect in the chlorination of toluene. Other objects and advantages will become apparent from the details and examples provided herein below.